1. Field of the Invention
The present invention relates to a galvannealed steel sheet superior in workability. The steel sheet excels in drawability owing to the retained austenite, and the alloy zinc plating excels in slidability. Incidentally, the term xe2x80x9cworkabilityxe2x80x9d used in this specification means mechanical characteristics, such as total elongation and tensile strength, and surface slidability important for press working.
2. Description of the Related Arts
A galvannealed steel sheet is required to have both high strength and high ductility if it is to be applied to automobiles and industrial machines by press working. This requirement is becoming stringent more and more in recent years.
Conventional steel sheets designed to have both high strength and high ductility include those of ferrite-martensite dual-phase steel and ferrite-bainite dual-phase steel. (The former consists of ferrite as the base material and martensite resulting from low-temperature transformation.) Of these steel sheets, the one which contains retained austenite subject to strain induced transformation during working is particularly useful. (This steel is referred to as TRIP (TRansformation Inducing Plasticity) steel hereinafter.) With a controlled structure such that the parent phase is ferrite and the second phase is bainite (which may contain martensite), the steel sheet containing retained austenite has improved ductility owing to retained austenite and ferrite and improved strength owing to bainite (or martensite resulting from strain induced transformation).
The steel sheet containing retained austenite is obtained usually by adding a large amount of silicon to steel. Silicon effectively prevents retained austenite from decomposing to form carbide and contributes to the formation of retained austenite. On the other hand, silicon is known as an element to inhibit alloying. Therefore, silicon added in a large amount retards alloying, causes incomplete plating, and deteriorates anti-powdering properties. One effective way to accelerate alloying is to raise the alloying temperature above 800xc2x0 C. or so; however, this is not desirable because retained austenite decomposes at such high temperatures.
Some ideas have been proposed to address the above-mentioned problems. For example, Japanese Patent Laid-open No. 131145/1999 discloses a high-strength high-ductility galvannealed steel sheet which contains retained austenite. According to the disclosure, it is possible to secure as much retained austenite as desired if a steel sheet is formed from a steel which has been incorporated with aluminum (which effectively forms retained austenite) in a large amount and silicon (which causes incomplete plating) in a controlled amount and which has been processed under minutely controlled conditions (for annealing, hot-dip zinc-coating, and alloying treatment). However, the present inventors"" investigation revealed that the disclosed procedure improves the elongation of the parent material but decreases the slidability of the plating surface in press working.
Likewise, Japanese Patent Laid-open No. 3150/2001 discloses a TRIP steel sheet containing retained austenite. However, the disclosed steel sheet does not exhibit invariably high elongation. A probably reason for this is that the amount of retained austenite fluctuates and the characteristic properties of retained austenite (e.g., the carbon content in the retained austenite) are not fully elucidated. In addition, the steel sheet in the second disclosure mentioned above has problems with the slidability of plating surface. Thus, its overall rating for workability is poor. The alloying process in the second disclosure consists of cooling a hot-dip zinc-coated steel sheet below 300xc2x0 C. at an average rate of 5-50xc2x0 C./s and then reheating it up to 450-550xc2x0 C. The disadvantage of this process (which does not include austemper) in inability to give retained austenite necessary for improvement in elongation. In addition, the disclosed process merely yields a galvannealed steel sheet from a low-silicon steel (containing 0.5-0.7 mass % silicon). So far, there is no process capable of yielding a steel sheet containing retained austenite from a high-silicon steel which has high elongation and improved surface slidability.
The present invention was completed in view of the foregoing. It is an object of the present invention to provide a new galvannealed steel sheet which is produced from a high-silicon steel containing retained austenite. The zinc-coated steel sheet has good mechanical properties (such as total elongation) and good plating surface slidability.
The gist of the present invention reside in a galvannealed steel sheet superior in workability wherein the steel sheet is characterized by containing silicon in an amount no less than 0.8 mass % and retained austenite in an amount no less than 3% (in terms of packing factor) and the alloy zinc plating is characterized by containing Znxe2x80x94Fe alloy crystal grains which exist in its surface layer and have a specific crystal grain size such that the longer side of a crystal grain is no larger than twice the shorter side of a crystal grain and the number of crystal grains with an average particle size no smaller than 4 xcexcm is no more than 5 per 70xc3x9750 xcexcm.
According to a preferred embodiment of the present invention, the steel may contain, in addition to silicon, the following components (in terms of mass %).
Al: 0.01-0.4%
C: 0.06-0.6%
Mn: 0.5-3%
P: no more than 0.15% (excluding 0%)
S: no more than 0.02% (excluding 0%)
According to another preferred embodiment of the present invention, the steel may additionally contain the following components (in terms of mass %).
(1) at least one species selected from:
Mo: no more than 1% (excluding 0%)
Ni: no more than 0.5% (excluding 0%)
Cu: no more than 0.5% (excluding 0%)
Cr: no more than 1% (excluding 0%)
(2) at least one species selected from:
Ti: no more than 0.1% (excluding 0%)
Nb: no more than 0.1% (excluding 0%)
V: no more than 0.1% (excluding 0%)
(3) at least one species selected from:
Ca: no more than 30 ppm (excluding 0 ppm)
REM: no more than 30 ppm (excluding 0 ppm).
The present inventors carried out investigations to solve the above-mentioned problems (such as slow alloying rate) involved in a high-silicon steel sheet containing retained austenite. As the result, it was found that a galvannealed steel sheet has greatly improved plating surface slidability even though it is made of high-silicon steel if the alloy zinc plating is so formed as to contain a limited number of coarse crystal grains of Znxe2x80x94Fe alloy in the surface layer of alloy zinc plating. The present invention is based on this finding.